Timer-controlled clamp for initiation elements

ABSTRACT

This invention relates to protective circuitry ( 12 ) for electrical initiation elements ( 10 ) and finds utility in preventing inadvertent functioning of electrical bridge-initiation elements, such as semiconductor bridges (SCBs), bridgewires, etc., by transient environmental electrical signals. The protective circuitry ( 12 ) of this invention comprises a timer portion ( 14 ) and a clamping portion ( 16 ) and is designed to divert from the electrical initiation element ( 10 ) at least a portion of an electrical signal received at the input nodes ( 10   a   , 10   b ) thereof for a suitable time interval that corresponds to the duration of an expected transient signal, which is typically significantly smaller than the duration of a proper initiation signal.

BACKGROUND OF THE INVENTION Field of the Invention

This invention relates to protection circuitry for electrical componentsand, in particular, to the protection of electrical initiation elementsfor use with reactive material, e.g., in squibs, detonators, and thelike.

SUMMARY OF THE INVENTION

This invention provides an initiator device comprising an electricalinitiation element having signal input nodes thereto with protectivecircuitry connected across the signal input nodes. The protectivecircuitry comprises a clamping portion responsive to input signals atthe input nodes to divert from the initiation element at least a portionof such input signals, the clamping portion being responsive to arelease signal to permit the input signal to pass to the initiationelement upon receipt of such release signal, and a timer portionconnected to the clamping circuit and to the input nodes, and beingresponsive to such input signals, for issuing a release signal to theclamping portion after passage of a clamping interval after the receiptof the input signal.

According to one aspect of the invention, the clamping interval may beabout 100 microseconds or less. For example, the clamping interval maybe in the range of from about 1 microsecond to about 100 microseconds,or from about 10 microseconds to about 100 microseconds.

In various embodiments of the invention, the initiator device maycomprise a unipolar clamping circuit and a unipolar timer circuit, or itmay comprise a bipolar clamping circuit and a bipolar timer circuit.

Optionally, one or both of the electrical initiation element and theprotective circuitry may be formed as integrated circuitry. For example,the initiation element and protective circuitry may be mounted on aheader comprising two electrical leads connected to the protectivecircuitry, and the device may further comprise a shell mounted on theheader and a charge of reactive material in the shell for initiation bythe initiation element.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of unipolar active clampingcircuitry with an electrical initiation element in accordance with thisinvention;

FIG. 2 is a circuit diagram of a particular embodiment of the circuitryof FIG. 1;

FIG. 3 is a schematic representation of bipolar clamping circuitryaccording to the present invention, with an electrical initiationelement;

FIG. 4 is a circuit diagram of a particular embodiment of activeclamping circuitry as represented in FIG. 3;

FIG. 4A is a schematic cross-sectional view of an initiator comprisingan electrical initiation element with protective circuitry according tothe present invention; and

FIGS. 5, 6 and 7 are plots of current flowing through a resistiveelement in place of an electrical initiation element as described in theExample, from which the clamping intervals are evident.

DETAILED DESCRIPTION OF THE INVENTION AND PREFERRED EMBODIMENTS THEREOF

This invention relates to protective circuitry for electrical initiationelements of the kind commonly used to initiate reactive effectors, i.e.,explosive or pyrotechnic devices such as initiators (squibs, detonators,etc.), exploding bolts, etc. The protective circuitry serves to preventinadvertent functioning of the initiation element in response to atransient environmental electrical signal while allowing the initiationelement to function in response to a proper initiation signal. Thecircuitry functions by diverting (“clamping”) from the electricalinitiation element at least a portion of an input electrical signal fora time interval (the “clamping interval”) that corresponds to theduration of a typical transient signal. In this way, transient signalsdo not initiate the initiation element. The clamping interval issignificantly smaller than the duration of a proper initiation signal,so that an adequate proportion of a proper initiation signal can pass tothe initiation element to make it function. The protective circuitry ofthis invention therefore has clamping circuitry and timer circuitry towhich the clamping circuitry is responsive and which determines theclamping interval.

The protective circuitry of this invention takes advantage of the factthat many transient pulses capable of causing the inadvertentfunctioning of an initiation element are much shorter in duration than abona fide initiation signal. The protective circuitry thereforefunctions by diverting away from the initiation element, for a shorttime interval (“the clamping interval”), at least a portion of any inputcurrent above a minimum threshold supplied to the initiation element.After the clamping interval, the input current is permitted to flow tothe initiation element. The clamping interval is selected to be longenough to block a typical transient signal, but not so long that thereliability of the response of the initiation element to a bona fideinitiation signal is significantly affected. The response of theinitiation element to the initiation signal is delayed by the clampinginterval, so the initiation signal must exceed the function time of theinitiation element by at least as much as the clamping interval.

In a typical embodiment, the protective circuitry for a semiconductorbridge (SCB) initiation element designed to have a function time of lessthan 500 μs in response to a 2 millisecond (ms), 1 ampere (A) initiationsignal may limit the bridge current to not more than about 0.5 A duringa clamping interval of up to about 100 microseconds (tes). Thus, theclamping interval may last for up to about 20% of the expected functiontime of the initiation element given the proper initiation signal and,in this example, up to about 5% of the duration of the initiationsignal. Preferably, the protective circuitry is designed to clamp inputsignals that fall within its no-fire limitation for the device, whichmay require that the SCB not fire in response to a rectangular 2.5 Ainput signal lasting 50 μs at 25° C., or a 5.3 A, 4 μs rectangularpulse. Typically, the clamping interval will be at least about 1 μs,preferably at least 10 μs.

Protective circuitry according to this invention comprises a clampingportion and a timer portion. A general representation of a unipolarembodiment of such protective circuitry is shown in FIG. 1, connected toan electric initiation element or “bridge” 10, which may be, e.g., anSCB, a titanium bridge, an exploding bridgewire, etc. The protectivecircuitry 12 comprises a timer portion 14 comprising a timer circuit anda clamping portion 16 comprising a clamping circuit, both of which arepowered by an input signal received at nodes 10 a and 10 b. Bridge 10receives initiation signals and, possibly, various undesired signalssuch as circuit transients, electrostatic discharge, etc., via nodes 10a and 10 b. The clamping portion 16 is connected across bridge 10 inparallel thereto relative to nodes 10 a and 10 b. In effect, clampingportion 16 comprises a switch which, when closed, creates a circuit inparallel with bridge 10 that diverts away from bridge 10 a significantportion of any current generated by a potential across nodes 10 a and 10b. The clamping portion 16 is responsive to the initial application of apotential across nodes 10 a and 10 b which, in the illustrated circuit,defines a potential across bridge 10. However, the operation of clampingportion 16 is controlled by timer portion 14, which disables theclamping portion 16 after a predetermined time interval (the clampinginterval) by generating a release signal that causes the clampingcircuit to release (i.e., stop clamping) the input signal. If apotential remains across nodes 10 a and 10 b after the clampinginterval, any current generated thereby will then flow through bridge 10and may cause bridge 10 to function.

A circuit diagram of the particular embodiment of the protectivecircuitry 12 of FIG. 1 is provided in FIG. 2. As shown in this Figure,the timer circuit of timer portion 14 comprises an RC circuit(resistance R1 and capacitor C1) together with transistors Q1 and Q2.The clamping circuit of clamping portion 16 comprises a shunt switchcomprising a resistor R2 and transistors Q3, Q4 and Q5. The operation ofsuch protective circuitry 12 is described in the Example below.

The protective circuitry shown in FIG. 2 is unipolar in nature, i.e., itwill function only in response to a potential across nodes 10 a and 10 bof a particular polarity. However, unwanted stray currents anddischarges across nodes 10 a and 10 b might also have the oppositepolarity to which the circuit of FIG. 2 cannot respond, leaving thebridge 10 vulnerable to inadvertent firing. For this reason, it ispreferred to provide bipolar active clamping circuitry to protectagainst the inadvertent function of an electrical initiation element.

A schematic representation of bipolar protective circuitry according toFIG. 1 connected across a bridge and nodes 10 a′, 10 b′ is shown in FIG.3. As shown in this Figure, timer portion 14′ comprises two timercircuits, timer circuit 122 a and timer circuit 122 b, each designed tooperate in response to an input signal of opposite polarity from theother. Clamping portion 16′ comprises two clamping circuits, eachcomprising a shunt switch that works in conjunction with a diode andwhich is designed to clamp signals of an opposite polarity from theother. Timer circuit 122 a controls a clamping circuit in clampingportion 16′ comprising a shunt switch 118 a that works in conjunctionwith diode 120 a. Timer circuit 122 a, shunt switch 118 a and diode 120a cooperate to provide an active clamping function across bridge 10 fora predetermined clamping interval in response to input signals of aparticular polarity. Conversely, timer circuit 122 b controls a clampingcircuit comprising a shunt switch 118 b and diode 120 b to provide theactive clamping function in response to signals of an opposite polarityfrom those to which timer circuit 122 a, etc., respond.

A circuit diagram of a particular clamping circuit according to theschematic of FIG. 3 is provided in FIG. 4. The circuitry of FIGS. 3 and4 include a zener diode portion 124 that protects the bridge, the timerportion and the clamping portion from high power transients such aselectrostatic discharges whose magnitudes and/or speed exceed theclamping ability and/or response time of the clamping circuit. The zenerdiode portion 124 comprises two zener diodes in series but in reversebias orientation relative to each other across nodes 10 a′ and 10 b′.

Optionally, a zener diode portion may also be employed in the unipolarembodiment of FIGS. 1 and 2 across nodes 10 a and 10 b, optionally witha suitably biased single diode.

The circuits represented in FIGS. 2 and 4 can easily be reduced topractice substantially as shown using discrete circuit elements.However, the preferred embodiment of the clamping circuits and of theelectrical initiation element with which they are used is in the form ofa solid state integrated circuit die having a solid state initiationelement (e.g., a semiconductor bridge (SCB), tungsten bridge, or thelike) formed on a suitable substrate. For example, as is well-known inthe art, an SCB die comprises a non-conductive substrate on which theSCB and optional associated circuit elements are formed. Such a die isformed with contact pads that provide input nodes to which lead wirescan be connected to provide the electrical initiation signal. Theprotective circuitry of this invention can be formed as integratedcircuitry on the die with the initiation element, or on a separate die,or from discrete circuit elements. In producing the integrated circuitembodiments, certain routine alterations to the illustrated circuitdiagrams would be made as a matter of routine to accommodate thedifferent characteristics and capabilities of circuit elements(resistors, capacitors, etc.) formed using integrated circuit technologyrelative to the characteristics of discrete circuit elements.

There is shown in FIG. 4A an initiator comprising an electricalinitiation element and associated protective circuitry in accordancewith this invention. Initiator 30 comprises an SCB die 32 comprising anon-conductive substrate (e.g., sapphire) on which is formed asemiconductor bridge initiation element and protective circuitry inaccordance with this invention, using integrated circuit manufacturingtechnology. Die 32 is secured to a header 34 by a thin layer of epoxy36. Header 34 and epoxy 36 are formed from non-conductive material.Input nodes for the protective circuitry and the semiconductor bridgeare provided by metallized lands 38 a and 38 b on the die. Electricalleads 40 are mounted in header 34 and are connected to metallized lands38 a and 38 b by lead wires 42 a and 42 b. A shell 44 containing areactive material 46 is secured to header 34 such that reactive material46 is in contact with the initiation element on die 32. The reactivematerial 46 may comprise an explosive charge, whereby upon thefunctioning of the semiconductor bridge, reactive material 46 willgenerate an explosive output from shell 44. Alternatively, reactivematerial 46 may comprise a pyrotechnic material that generates apyrotechnical output.

EXAMPLE

A prototype clamping circuit according to the circuit diagram of FIG. 2was assembled and tested on a breadboard using commercially availableparts. Specifications for the circuit elements used in the circuit areas follows. C1 100 pF Ceramic Capacitor, 10%, 100 V Q1-3 2N2222A NPNSmall Signal Transistor Q4 2N2907A PNP Small Signal Transistor Q5MMJT9410T1 NPN Medium Power Transistor R1 249 k′Ω, 1%, 0.1 W, Metal FilmR2 10 k′Ω, 1%, 0.1 W, Metal Film R_((SCB)) Two 1 ′Ω, 1 W, Wirewound inseries (simulates a standard SCB)

Transistors Q1-Q5 were bipolar junction transistors with a beta of about75, preferably at least about 50. Transistor Q5 must be capable ofhandling large currents (e.g., about 1 ampere (A)) with a low V_(CE).Upon the application of a simulated input current transient, a voltagedeveloped across R_((SCB)) (which is a nominal 2 Ω resistance) (Fortesting purposes, a resistor is used in place of an SCB or otherelectric initiation element.). Until capacitor C1 charges sufficientlyto activate transistor Q1, both transistors Q1 and Q2 are held in the“off” state. This allows current to flow through resistor R2, providingbase drive to transistor Q3. The transistor Q3 collector currentprovides base drive to transistor Q4, which in turn provides base driveto transistor Q5, which shunts at least a portion of the input currentaway from resistor R_((SCB)).

The timer circuit operates by delaying the turn-on of transistor coupleQ1/Q2 until capacitor C1 has charged sufficiently to activate transistorQ1. At that point, transistor Q1 turns on and provides base drive totransistor Q2. Transistor Q2, when on, effectively generates a releasesignal that clamps the base-emitter voltage of transistor Q3, whichturns off transistors Q4 and Q5, allowing substantially all of theremaining input current to flow through resistor R_((SCB)). Due to thelarge current gain, the collector-emitter saturation voltage V_(CE(SAT))of transistor Q3 should always be less than V_(CE(SAT)) of transistorQ4, and, similarly, the base-emitter voltage V_(BE) of transistor Q4should always be less than V_(BE) of transistor Q5.

The circuit contains hysteresis when the clamp turns off. Once thecapacitor C1 voltage is large enough to turn on transistor couple Q1/Q2,the clamp begins to turn off Assuming a constant input current, thecurrent through resistor R_((SCB)) begins to rise as the clampingcircuit turns off, resulting in a larger voltage across resistorR_((SCB)). This positive feedback, together with the high circuit gain,produces a fast turnoff of the clamping circuit. This should be kept inmind in the event resistor R1 is replaced by a current source to reducethe delay variation caused by variation in the input current. A currentsource embodiment of that kind is not expected to have such a dramaticpositive feedback.

Inherently, the particular circuit elements chosen for the timer circuitand the clamping circuit will make those circuits unresponsive to inputsignals of less than a threshold magnitude, thus providing an inherentthreshold sensing function to the protective circuitry. For example, theclamping circuit will not function unless the input signal generates acurrent in R2 sufficient to activate transistor Q3. Similarly, the timercircuit transistor couple Q1/Q2 will not turn on until the input voltageexceeds their combined V_(BE) thresholds. In this example, this meansthat there will be a range of input currents between 0.5 A and 0.6 Awhere the clamp will most likely turn on, but may not turn off. Theprotective circuitry is easily designed by one of ordinary skill in theart so that the thresholds are below the magnitude of expected transientsignals capable of causing the inadvertent functioning of the initiationelement.

Test data for the breadboard circuit are shown in Table 1 for 50microsecond current pulse. The input current is the current into theinput nodes of the circuit and the bridge current is the currentmeasured through resistor R_((SCB)). TABLE 1 Input Current (A) CurrentThrough R_((SCB)) 0.50 0.42 0.75 0.46 1.00 0.52 1.25 0.54 1.50 0.56 1.750.58 2.00 0.60 2.25 0.62 2.50 0.64

The data of Table I show that at the smallest test current of 0.5amperes (A), the clamp has already begun to turn on and divert about 80mA from the bridge. At an input current of 1.0 A the clamp is shuntingabout one-half of the input current. At the maximum tested transientlevel of 2.5 A, the clamp shunts about 1.86 A with the remaining 0.64 Aflowing through the bridge.

Further testing was done at a normal firing input of 1.2 A and up to 2.0A. The resulting waveforms of current through R_((SCB)) are shown inFIGS. 5, 6 and 7. In these Figures, the horizontal axis is graduated in50 μs intervals and the vertical scaling is 0.5 A/division. FIG. 5 showsthat, in response to an input signal of 1.2 A, the current received atR_((SCB)) was merely 0.5 A for a clamping interval of about 100 μs.Thereafter, the timer released the clamp and the voltage climbed to 1.2A, i.e., the full input current was received through R_((SCB)). FIG. 6shows that, in response to an input signal of 1.5 A, the currentreceived by R_((SCB)) was reduced by the clamping circuit so that acurrent of merely approximately 0.5 A was developed for a clampinginterval of about 75 μs. After the clamping interval, the current atR_((SCB)) attained the full 1.5 A. FIG. 7 shows that, in response to aninput signal of 2 A, the attenuated current at R_((SCB)) during theclamping interval was merely about 0.5 A and that the clamping intervallasted about 50 μs. Thereafter, the timer released the clamping circuitand the full input current was supplied to R_((SCB)), producing acurrent of 2 A. The clamping interval becomes predictably shorter as theinput current becomes greater, because the greater input current chargescapacitor C1 more quickly. However, one skilled in the art, once giventhe foregoing disclosure, could design active clamping circuitry inaccordance with this invention suitable for diverting current from thebridge for any designated clamping interval.

Although the invention has been illustrated and described with respectto two particular embodiments thereof, it will be understood by one ofordinary skill in the art upon a reading of the foregoing disclosurethat numerous alterations and variations of those embodiments fallwithin the scope of the invention and the following claims.

1. An initiator device comprising: an electrical initiation elementhaving signal input nodes thereto; protective circuitry connected acrossthe signal input nodes, the protective circuitry comprising a clampingportion responsive to input signals at the input nodes to divert fromthe initiation element at least a portion of such input signals, theclamping portion being responsive to a release signal to permit theinput signal to pass to the initiation element upon receipt of suchrelease signal; and a timer portion connected to the clamping portionand to the input nodes, and being responsive to such input signals, forissuing a release signal to the clamping portion after passage of aclamping interval after the receipt of the input signal.
 2. Theinitiator device of claim 1 wherein the clamping interval is about 100microseconds or less.
 3. The initiator device of claim 2 wherein theclamping interval is in the range of from about 5 microseconds to 100microseconds.
 4. The initiator device of claim 2 wherein the clampinginterval is in the range of from about 20 microseconds to 100microseconds.
 5. The initiator device of claim 1 comprising a unipolarclamping circuit and a unipolar timer circuit.
 6. The initiator deviceof claim 1 comprising a bipolar clamping circuit and a bipolar timercircuit.
 7. The initiator device of claim 1 wherein at least one of theelectrical initiation element and the protective circuitry are formed asintegrated circuitry.
 8. The initiator device of claim 7 wherein theinitiation element and protective circuitry are mounted on a headercomprising two electrical leads connected to the protective circuitry,and further comprising a shell mounted on the header and a charge ofreactive material in the shell for initiation by the initiation element.